Leaching and Recovery of Molybdenum from Spent Catalysts

Spent catalysts are generated in large quantities as solid waste on a yearly basis. Owing to their hazardous nature, environmental regulations for discarded spent catalysts are very strict. Consequently, from the ecological and economical viewpoint, metal recovery from spent catalysts is very important. Molybdenum (Mo) is one of the elements present in spent catalysts at high concentrations. The rapidly growing demands for Mo and its products create a need to develop novel recovery processes from secondary resources, i.e. spent catalysts, as there is a projected shortage of the primary resources for this element. This chapter overviews: (i) the general characteristics of Mo along with the research conducted in recent years concerning Mo chemical leaching, i.e. with strong inorganic acids, sodium hydroxide, hydrogen peroxide and acid mixtures, (ii) Mo bioleaching, i.e. with bacteria Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans and Acidianus brierleyi or fungi Penicillium simplicissimum and Aspergillus niger and (iii) Mo recovery, i.e. precipitation, adsorption, ion exchange and solvent extraction, from spent catalysts. Chemical leaching offers Mo leaching yields exceeding 90%. Sulphuric acid seems to be the best leachant owing to its high oxidising power. Bioleaching offers more cost-efficient, simpler and more environmentally friendly processes. However, bioleaching has relatively long leaching cycles (usually around 20 days) and extraction efficiencies of Mo are below 70%. Among the recovery techniques, solvent extraction is well-established in hydrometallurgy for commercial production of high purity Mo.